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Title: Screwworms, Cochliomyia hominivorax, reared for mass release do not carry and spread foot-and-mouth disease virus and classical swine fever virus

Author
item Chaudhury, Muhammad
item WARD, GORDON - USDA, APHIS, VS, FADDL
item Skoda, Steven
item DENG, MING - USDA, APHIS, VS, FADDL
item Welch, John
item MCKENNA, TOM - USDA, APHIS, VS, FADDL

Submitted to: Journal of Insect Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/22/2008
Publication Date: 10/23/2008
Citation: Chaudhury, M.F., Ward, G.B., Skoda, S.R., Deng, M.Y., Welch, J.B., McKenna, T.S. 2008. Screwworms, Cochliomyia hominivorax, reared for mass release do not carry and spread foot-and-mouth disease virus and classical swine fever virus. Journal of Insect Science. 8:62.

Interpretive Summary: The screwworm eradication program uses the sterile insect technique (SIT) to eradicate this pest of veterinary importance. SIT depends on quality insects that are reared, sterilized, and released in the field to achieve eradication. Periodically, new laboratory strains of screwworm are developed for this purpose; screwworms are field collected from various countries where this pest is still available. It was questioned whether transporting live screwworm, from countries where foot-and-mouth disease (FMD) and classical swine fever (CSF) are present, to the mass rearing facilities in Mexico and Panama may introduce these exotic diseases. We investigated this by growing screwworms in a culture that was inoculated with either the FMD virus or the CSF virus. The culture medium was treated with either antibiotics or formalin to prevent fungus and bacterial growth that otherwise normally occurs. Insects growing in these cultures were sampled daily for 5 days and checked for the presence of virus using appropriate procedures. Samples of culture media alone were also taken and checked for viruses. Only FMD virus was detected in the insects of day 1 samples and culture samples of day 1, 2, and 3 for the cultures with antibiotics. For the cultures containing formalin, neither virus was found in insect samples or in culture samples of any day; indicating that the viruses can not survive in insects grown in the culture containing formalin. Therefore, screwworm materials collected in countries endemic for FMD and/or CSF and reared in formalin-containing medium for at least one generation at the site of the collection should be virus free. These results are important for continued success of the screwworm eradication program. Following the rearing protocol established from these results, screwworm materials can be transported safely to the rearing facility in Mexico or Panama for the development of new strains and mass rearing for the eradication program.

Technical Abstract: Transporting live screwworms Cochliomyia hominivorax Coquerel for developing new strains from countries where foot-and-mouth disease (FMD) and classical swine fever (CSF) are endemic, to the mass rearing facilities in Mexico and Panama may introduce these exotic diseases. This study was conducted to investigate if screwworms are capable of harboring and spreading FMD virus (FMDV) and CSF virus (CSFV) when they are grown in virus-inoculated larval rearing medium. In one experiment, screwworm larvae were reared in a FMDV-inoculated artificial medium containing either 0.1% formaldehyde or antibiotics as an antimicrobial agent. In another experiment, larvae were similarly reared in a CSFV-inoculated artificial medium containing 0.1% formaldehyde. In each experiment, samples of larvae were collected from the medium until pupation and samples of the medium were also collected. Viruses were extracted from the larvae and the media using standard procedures. The detection of FMDV was done by observing cytopathic effect on cell cultures and a conventional reverse transcription-polymerase chain reaction (RT-PCR); the detection of CSFV was done by an Avidin-Biotin Complex (ABC) assay and a conventional RT-PCR. For media containing antibiotics, FMDV was detected in a larval sample collected on day 1 and in media samples of days 1, 2 and 3; no FMDV was detected from larval and media samples collected on all other days. For media containing formaldehyde, both FMDV and CSFV were not detectable in larval or media samples collected on all sampling days. These results indicate that the FMDV and CSFV cannot survive in the rearing medium which contains formaldehyde as an antimicrobial agent. Therefore, insects collected in endemic regions and reared in the formaldehyde-containing medium for at least one generation (from egg to pupal stage) at the collection site should be free of FMDV and CSFV and could then be transported safely to the strain development/mass rearing facility.